The present invention relates to an infrared imaging device, and more particularly to a thermal-type infrared imaging device which detects an incident infrared ray as heat.
The thermal-type infrared imaging device is used, for example, for the purpose of measuring the surface temperature distribution of an object by absorbing infrared rays irradiated from each part of an object and converting them to heat, furthermore converting heat to an electric signal, and displaying the signal as an image.
In the thermal-type infrared imaging device, it is necessary to filter out noise to enhance a signal-to-noise (S/N) ratio and also to make a fluctuation between pixels smaller.
A conventional thermal-type infrared imaging device is described, for example, in Japanese Patent Application Laid-Open No. 7-193752 and Japanese Patent Application No. 6-189144, which are inventions of a prior application of the present inventor.
FIG. 21 shows a sectional view of a conventional thermal-type infrared imaging device, and FIG. 22 shows a plan view of the conventional thermal-type infrared imaging device. The thermal-type infrared imaging device according to the prior application, as shown in FIGS. 21 and 22, has a semiconductor substrate 20 and a scanning circuit 21 on the substrate, and on the scanning circuit 21, has a light receiving section for converting an incident infrared ray to an electric signal.
The light receiving section has a silicon oxide film 22, a cavity 23, a ground wire 24 consisting of aluminum (Al), a signal wire 25 consisting of Al, a slit 26, a titanium bolometer 27, a silicon oxide film 28, titanium nitride 29, and a vertical selection line 30.
The scanning circuit and the light receiving section integrate a plurality of circuits and light receiving sections with respect to a pixel so that a two-dimensional infrared image can be obtainable.
The light receiving section comprises an infrared-ray absorbing layer for absorbing an infrared ray, a diaphragm for preventing escape of heat, and a thermo-electric converting element for converting heat to an electric signal. The diaphragm forms a floating film structure by removing the underlying layer by etching. The thermo-electric converting element in this example employs a bolometer where the electric resistance value varies with temperature and employs titanium as material of the bolometer.
The infrared ray incident on each pixel is absorbed by the infrared-ray absorbing layer of each pixel and causes the temperature of the diaphragm of each pixel to rise. This temperature rise is converted to an electric signal by the titanium bolometer, and the electric signal is output in sequence to an external circuit through the circuit on the substrate. Note that the details of these are described in No. 6-189144.
Objects irradiate an infrared ray having a certain power corresponding to the temperature, based on Plank's equation. For this reason, the infrared imaging device has a large bias component unlike an imaging device for visible light. For example, when the infrared ray of an object of near 300.degree. K. is imaged, there is the need to take out a slight signal component which is above a large bias component irradiated by an object of 300.degree. K.
Furthermore, in the infrared imaging device of the bolometer type, it is necessary that a bias current flows in order to read out a signal. In the infrared imaging device of the bolometer type which operates at normal temperature, the rate of a bias current becomes larger particularly to a signal current and this is also causative of making the bias component larger.
On the other hand, a conventional thermal-type infrared imaging device is provided with a constant current source in addition to a transistor which converts a signal to a current, and a very large bias component is canceled with respect to this signal.
FIG. 23 shows a circuit diagram of the conventional thermal-type infrared imaging device. The thermal-type infrared imaging device has a bolometer 1, a pixel switch 2, a vertical AND element 3, a horizontal switch 4, a horizontal AND element 5, a ground wire 6, a vertical signal line 7, a horizontal signal line 8, a vertical shift register 9, a latch 10, a horizontal register 11, a first output 12, a second output 13, a first integration circuit 14, and a second integration circuit 15. Note that the details of these are described in No. 7-193752.
In addition, in another example an integration capacitor is provided in each pixel. With this, integrating time can be made larger, and the band of noise is narrowed and noise can be reduced (see No. 7-193752). As another example, a quantum-type infrared imaging device is described in Japanese Patent Application Laid-Open No. 7-87406 (Japanese Patent Application No. 5-229946). An integration capacitor is provided in each line so that it can be made larger.
Generally, in the infrared imaging device there exists a fluctuation in the bias level which results from a fluctuation in the detector of each pixel. This is referred to as fixed pattern noise (FPN), and a correction circuit is usually provided to correct the noise. As a conventional example, memory for holding the fluctuation quantity of a bias level is provided exteriorly of an infrared imaging element, and fixed pattern noise is corrected (see No. 6-189144).
Also, in the example of the prior application, the output of a device has an integration transistor, the emitter of the transistor is connected to the output, and the collector is connected to an integration capacitor (see No. 6-189144).
The technique, disclosed in No. 7-193752 of the related application can filter out a large bias component and take out a signal component, but it has the following problems.
In order to integrate the signal of the thermo-electric converting element, there is the need to convert the resistance change or voltage change of the thermo-electric converting element to a change in the current. For this reason, an amplifying element, such as a transistor, or a nonlinear element becomes necessary. However, transistors have noise such as shot noise and Johnson noise regardless of the transistor type such as the bipolar type and the MOS type. In order to improve the S/N ratio, the noise caused by the transistor needs to be made as smaller as possible.
The integration circuit, used in No. 6-189144, has very low noise, but there is no consideration for the aforementioned canceling of the bias component. Even if the techniques of both prior applications were combined with each other, the combination would further have the following problems.
In a thermal-type infrared imaging device using a bolometer, such as that shown in FIG. 22, when the temperature of the device changes, the temperature of the bolometer on the device also changes and therefore the temperature change, as it is, will appear in a signal. For example, when an object with a temperature difference of 1.degree. C. is viewed, the temperature change of the diaphragm is a slight temperature difference of about 0.002.degree. C. and therefore the temperature change (drift) of this device gives a large influence. Therefore, to cope with this, a constant temperature device and a correction circuit become necessary, and in addition, there arises the problem that the dynamic range of a signal is narrowed.
Furthermore, the noise of the circuit, which cancels a bias component, needs to be made as small as possible for an improvement in the S/N ratio. However, in the conventional example there is no consideration for this.
Moreover, the conventional thermal-type infrared imaging device has an integration capacitor for each pixel, so there is a limit to an enlargement in the capacity of a capacitor. For this reason, there arises the problem that the dynamic range of a signal is restricted. On the other hand, there is an example of the quantum type where an integration capacitor is arranged in each column of pixels, but the quantum type, as it is, is not applicable to the thermal-type infrared imaging device.
For an improvement in an S/N ratio, there is the need to make bias current larger. In such a case, the operating voltage of a device is increased. However, a countermeasure in the case has not been considered in the conventional example.
If current flows through a bolometer, the self-heat generation of the bolometer will take place and there will be cases where the dynamic range of a signal will be restricted. However, a countermeasure in the case has not been considered in the conventional example.
The aforementioned fixed pattern noise has the problem that the dynamic range of a signal is narrowed, and therefore, in the conventional example a correction is made, for example, by a correction circuit. However, there arises the problem that the correction circuit requires a large-scale circuit such as an A/D converter and memory.